Delivery of a high-energy pulse during the vulnerable period of the cardiac cycle can induce ventricular fibrillation (VF) in patients. The vulnerable period encompasses the repolarization phase of the myocardial action potential, also referred to as the “recovery phase”, and a period immediately following it. The repolarization phase is observed as the T-wave portion of a cardiac ECG or EGM. During the vulnerable period, the ventricles are in an inhomogeneous state where certain regions are excitable and certain regions are refractory to stimuli. Delivery of a stimulation pulse, or “T-shock”, during this inhomogeneous state can initiate disorganized depolarization wave fronts causing fibrillation.
Patients undergoing implantation of an implantable cardioverter defibrillator (ICD) generally undergo electrophysiological testing to determine if the minimum shock energy required to terminate VF, referred to as the defibrillation threshold (DFT), meets the implant requirements for a particular ICD and lead configuration. In past practice, determination of the defibrillation threshold in a patient typically involved delivering a T-shock during the vulnerable period to induce VF and delivering a defibrillation shock there after to terminate the induced VF. A series of defibrillation shocks increasing or decreasing in energy can be delivered to determine the lowest energy that successfully defibrillates the heart.
A maximum T-shock energy exists, however, above which a T-shock pulse will not induce VF, even when delivered during the vulnerable period. The minimum T-shock energy at which VF induction does not occur is referred to as the “upper limit of vulnerability.” The upper limit of vulnerability (ULV) has been shown to be a predictor of the defibrillation threshold in a patient. Determination of the ULV could be substituted for defibrillation threshold testing at the time of ICD implantation. Generally, the implanting physician only needs to know if the patient meets the ICD implant criteria, i.e. if the patient's defibrillation threshold is acceptably below the maximum defibrillation shock energy available from the ICD. A clinician may select a shock energy that would be an acceptable DFT for a particular ICD and lead configuration. If VF is not induced by a T-shock delivered at the selected shock energy, the energy is assumed to be at or above the ULV for that patient. The clinician can therefore conclude that the selected shock energy is at or above the patient's DFT and thereby make the determination that the patient meets the ICD implant criteria. Using ULV measurements, a determination that a patient meets ICD implant criteria may be made by delivering as few as one T-shock without actually inducing VF. Such methods potentially improve the safety of ICD implantation procedures since actual VF induction may be avoided.
A T-shock that is less than the ULV will normally induce VF in susceptible patients when it is properly timed during the vulnerable period. However, such a T-shock delivered outside the vulnerable period may not induce VF, potentially misleading a clinician to think the T-shock energy is greater than the ULV. In order to properly couple the T-shock to the vulnerable period, a T-shock is typically delivered following a train of pacing pulses delivered at a rate greater than the patient's intrinsic heart rate. The T-shock is delivered following the last pacing pulse at a coupling interval that corresponds to a previously measured time interval between a pacing pulse and a subsequent T-wave. If all of the pacing pulses in the pulse train capture the heart, the pace-T-wave interval will be consistent and a T-shock delivered at that interval following the last pacing pulse will fall into the vulnerable period.
However, if one or more pacing pulses do not capture the heart, or if an intrinsic event occurs prior to T-shock delivery, the timing of the vulnerable period may change relative to the last pacing pulse of the pacing train. The T-shock may fail to induce VF irrespective of its amplitude. Without recognizing that the ventricular response to the pacing train has changed, a clinician may inappropriately conclude that the T-shock energy is above the patient's ULV. Inappropriate ULV determination may cause a clinician to determine that a patient's DFT is lower than it actually is and that the patient meets ICD implant criteria when he/she may not. Methods are needed for promoting reliable T-shock delivery during the vulnerable period in order to take advantage of using ULV determination during ICD implantation procedures.